feat(JSFunctionProxy): implement passing kwargs as a final argument - #506

This implements passing kwargs to JS functions, by appending the kwargs dict to the end of the arguments array, to have the same behaviour as pyodide.

So for example:

print(pm.eval("(function() {return [...arguments]})")(1, 2, i=42, j=43))
# [1, 2, {"i": 42, "j": 43}]

It also fixes a bug where passing kwargs to pm.new would result in an inscrutable error.